With a method of and an apparatus for manufacturing film of the type described above, such film is manufactured by performing a simultaneous biaxial orientation by expanding a tube-shaped film so as to make it in the form of a bubble shape by introducing a gas such as air into heated-thermoplastic resin between two sets of nip rolls each having an individual circumferential speed.
A conventional tubular method of manufacturing biaxially oriented films is schematically illustrated in FIG. 10. Referring to this figure, thermoplastic resin which has been supplied from a hopper 1 to an extruder 2 is heated and rendered molten by this extruder 2 so that the thus-molten thermoplastic resin is extruded from an annular extruding die 3. A heater for controlling the temperature of the molten resin to be extruded from the extruding die 3 is disposed on the outer surface of this extruding die 3.
A gas outlet portion 4 for introducing a gas (for example, air) into the molten tubular resin to be discharged from the extruding die 3 is provided within the extruding die 3, the gas being arranged to be supplied from a pump or the like (omitted from illustration) under a certain pressure.
When the molten tubular resin is continuously extruded from the extruding die 3 and the gas is simultaneously introduced from the gas outlet portion 4 into this tubular resin, the tubular resin is extruded in the form of a tube from the extruding die 3.
On the other hand, an air ring 5 and a cooling device 6 for supplying water whose temperature has been cooled down to a desired level are disposed at a certain interval from each other in the direction in which the tubular resin is extruded from the extruding die 3, and are also respectively maintained a predetermined distance from the extruding die 3. As a result, the thus-extruded tubular resin is cooled down by air sprayed by the air ring 5 and cooling water supplied from the cooling device 6 so that the tubular resin becomes a tubular original film A.sub.1 having a predetermined thickness and an outer diameter.
The front end portion of the original film A.sub.1 which has passed through the cooling device 6 is pressed by a nip roll 8 as a result of the introducing of the original film A.sub.1 by means of a guide plate 7, so that air can be enclosed within the tubular original film A.sub.1.
The original film A.sub.1 which has passed through the nip roll 8 becomes a folded original film A.sub.2, and this folded original film A.sub.2 is transmitted to a nip roller 11 via guide rolls 9 and 10.
A preheater 12 is provided, if necessary, on the conveyance line through which the folded original film A.sub.2 which has passed through the nip roll 11 is conveyed. In addition, an air ring 13 and a ring heater 14 are respectively provided with a predetermined interval maintained therebetween. As a result, the folded original film A.sub.2 which has passed through the nip roller 11 is heated by the ring heater 14, and a desired quantity of gas is forcedly injected into the folded original film A.sub.2 by an air injecting means (omitted from illustration) so that the folded original film A.sub.2 is expanded to form a bubble shape.
As a result, the portion in the vicinity of the front end portion of the bubble-shaped oriented film A.sub.3 is introduced into a nip roll 16 after it has passed along a flat guide roll 15 and has been given a desired hardness and an outer diameter. At this time, since the circumferential speed of the nip roll 16 is higher than that of the nip roll 11, the bubble-shaped oriented film A.sub.3 is formed as a result of a simultaneous and biaxial orientation performed in association with the operation of the thus-introduced air under a pressure.
The bubble-shaped oriented film A.sub.3 which has passed through the nip roll 16 is introduced into a heating furnace 18 via a conveying roll 17 in the form of a folded oriented film A.sub.4 wherein it is subjected to a final heat treatment. Then, thus-treated folded oriented film A.sub.4 is introduced into a conveying roll 19 so that it becomes an oriented film B. Next, this oriented film B is wound to a winding roll so as to be accommodated with the two side ends thereof having been opened by cutting.
Air to be injected from the air ring 13 is arranged to form an angle .alpha. with respect to the central axis of the bubble-shaped oriented film A.sub.3. As a result of the thus-arranged structure, the point at which formation of the bubble-shaped oriented film A.sub.3 starts is properly stabilized.
In order to provide uniform thickness of the oriented film B, the tubular original film A.sub.1 and the bubble-shaped oriented film A.sub.3 need to have a uniform thickness respectively. In order to achieve this, a structure is conventionally employed in which the width of a lip provided for the extruding die 3, that is the width of an outlet port through which the molten tubular resin is discharged, is arranged to be varied and this width (the degree of lip opening) is adjusted by a multiplicity of adjusting bolts.
However, a local adjustment by using the adjusting bolts can be difficult to perform since the extruding die 3 is designed to have an annular shape.
That is, if the width of a certain lip were reduced, the width of the other lip is necessarily widened. It will therefore be impossible to perform an adjustment that does not influence the overall arrangement. In other words, it involves a certain limitation in improving the thickness accuracy of the tubular original film A.sub.1 by such an adjustment, and the accuracy that can be realized is within an insufficiently narrow range of .+-.2 to 6%.
On the other hand, since the tubular original film A.sub.1 is expanded so as to be a bubble-shaped oriented film A.sub.3, the fine adjustment performed with the extruding die 3 is amplified by 1.5 to 4 times and influences the thickness distribution of the bubble-shaped oriented film A.sub.3. Therefore, the thickness distribution of the final product, that is, the oriented film B, is difficult to maintain uniform.
As a result, a serious problem arises when used as an industrial purpose film, such as defective appearance of a roll formed by winding the elongated biaxially oriented film, and defects generated at a secondary work such as printing, laminating and bag manufacturing.
Therefore, although the tubular biaxially orienting method exhibits an excellent advantage of a sufficient uniformity in the biaxial directions, the insufficient thickness accuracy involved therein prevents the wide use of this tubular biaxially orienting method.
In addition, since the adjustment by using the adjusting bolts can be varied depending upon the operation conditions of the overall manufacturing apparatus, working environment or the like, it needs to depend upon skilled workers. As a result, the following problems arise: apparatus automation is difficult to realize and working efficiency is insufficient.
Therefore, attempts have been made to improve the accuracy in the thickness distribution by the following techniques.
(1) A technique in which a heating cylinder is rotated in one direction or in a reciprocated manner so that the appearance of the roll is improved (see Japanese Patent Publication No. 47-28696).
(2) A technique in which the temperature of air to be sprayed through a hot air outlet port divided into sections along the circumference is adjusted on the basis of the displacement of the tube which has not as yet been oriented or which is being oriented (see Japanese Patent Publication No. 57-30369).
(3) A technique employed in a case where flat polyester films are oriented in the widthwise direction and arranged in such a manner that a heating device whose heating temperature distribution can be varied in the widthwise direction is provided in a region in which the magnification of orientation exceeds two, so that the heating temperature is raised when a portion of the film having a large width is heated (see Japanese Patent Laid-Open No. 52-47070).
However, the problem of nonuniform thickness cannot be essentially overcome by the technique (1), since the nonuniform thickness exceeds .+-.10% as described in an embodiment of the disclosure, although the appearance of the roll can be improved.
With the technique (2), since the structure is so arranged that hot air is used, that is, air is heated, heating in a divided manner cannot be correctly performed even if the heating air is divided into sections, certain limits being involved. In addition, as can be clearly seen from the embodiment disclosed, the structure in which a temperature difference is provided by performing a preheating before the film is oriented cannot assure the thus-provided temperature difference to be kept at the time of performing the orient.
Furthermore, with the technique (3), since the heating conditions required at the widthwise ends of the flat film are different from the heating conditions required at the central portion of the same, a satisfactorily control cannot be performed, causing the thus-performed control to become discontinued.
In order to obtain a film of uniform properties such as thickness, the shape (outer diameter) of the bubble-shaped oriented film A.sub.3 must be kept constant.
There is therefore known art (see Japanese Patent Publication No. 46-15439) wherein the flat guide roll 15 of FIG. 10 is arranged to be able to be opened/closed relative to its base portion adjacent to the nip roll 16 to form a sector. The outer diameter of the bubble-shaped oriented film A.sub.3 can be detected, and the flat guide roll 15 is opened/closed on the basis of the thus-detected outer diameter of the bubble-shaped oriented film A.sub.3, and control is thereby so performed that the outer diameter of the bubble-shaped oriented film A.sub.3 becomes constant. As a result, a rather uniform property oriented film can be obtained.
However, since the bubble-shaped oriented film A.sub.3 can be moved and the shape of it is not always a true circle, in the above-described structure in which the operation of the flat guide roll 15 is controlled, the outer diameter of the bubble-shaped oriented film A.sub.3 cannot be detected exactly. This leads to a result that the control of the flat guide roll 15 cannot be performed properly.
In addition, it is difficult to considerably change the inner pressure of the bubble-shaped oriented film A.sub.3 only by controlling the operation of the flat guide roll 15. Therefore, the quantity of the gas to be injected under pressure and enclosed within the bubble-shaped oriented film A.sub.3 must be maintained at an extremely correct level. However, the gas which has been thus injected under pressure will gradually flow out during the operation of the apparatus. This gives rise to a necessity to stop the operation of the apparatus after a certain quantity of the thus-enclosed gas has flowed out in order to start forming the bubble-shaped oriented film A.sub.3 again by enclosing the gas. Therefore, a continuous stable operation cannot be performed, and an excessive amount of resin and film are lost.
Furthermore, a disadvantage is involved that when the flat guide roll 15 is moved to form a sector, the angle at which the bubble-shaped oriented film A.sub.3 is flattened is changed. As a result, the manner in which the bubble-shaped oriented film A.sub.3 is conveyed can be changed, causing a problem in that the film A.sub.3 may be damaged.
In particular, since a controllable region of the capacities of each of the bubbles is extremely narrow in the conventional method, the formation of the bubbles by injecting air at the time of starting the orienting of the film must be performed precisely. Therefore, such operation cannot be readily performed in a large sized apparatus.
In the method of manufacturing the biaxially oriented film, a certain heat treatment is conducted for the purpose of stabilizing the dimensions of the film by fixing the orientation of the film molecules.
However, when nylon-6, which is a polyamide film, is subjected to a heat treatment nearly at 200.degree. C. with this tubular oriented film A.sub.3 folded to form a flat shape, the upper film and the lower film can be inevitably adhered to each other, and the thus-adhered two films cannot be separated from each other after the heat treatment. Therefore, the thus-adhered films will lose quality as a product. The problem of the type described above will be apparent in the case of a crystal thermoplastic resin film made of the polyamide.
In view of the foregoing, a tube method has been disclosed, for example, in which the oriented film folded to form a flat shape is subjected to a heat treatment, and an oven method has been also disclosed wherein the two sides of the oriented film are cut so as to make the oriented film two films, the thus-obtained two films are introduced into a tenter with a space held between the two films by an interposed endless belt, and the thus-introduced films are subjected to a heat treatment with the two ends of the films held by clips (see Japanese Patent Publication No. 46-15439).
In accordance with the above-described tube method, the adhesion of the two films can be prevented due to the presence of air between them. However, if a heat treatment exceeding 180.degree. C. were applied to the film in order to obtain an excellent stability upon the dimensions of the same, the bubbles can be staggered, causing a stable heat treatment to become impossible to be performed. As an alternative, a low temperature heat treatment cannot provide a sufficiently stable dimension stability.
On the other hand, in accordance with the oven method, the following problems are involved:
(i) Since the apparatus for interposing the endless belt between the films needs to become a large scale apparatus, disadvantages in view of the working space and cost for apparatus installation arise.
(ii) The film can be easily damaged.
(iii) The mechanism for holding the two ends of the film with the endless belt interposed therebetween becomes too complicated.
(iv) If the holding mechanism were not formed strong, the film can be separated from this holding mechanism, and a continuous treatment becomes impossible to perform because the contraction stress at the time of performing the heat treatment is too large.